Code 90 = Combustion abnormality. The flame characteristic during operation falls outside the controller's expected envelope — too rich, too lean, unstable. The unit shuts down to prevent unsafe combustion.
Diagnostic sequence
1. Vent restriction
Most common cause. Bird nest, ice, debris in vent termination, undersized vent for unit BTU. Inspect both intake and exhaust terminations.
2. Gas pressure
Below the minimum 4" WC NG / 10" WC propane during fire = lean combustion. Use a manometer; have a plumber check.
3. LP / NG conversion mismatch
If a unit was converted between NG and LP without the proper conversion kit and orifice change, combustion characteristic will be wrong. Verify the unit's gas orifice matches the gas type label.
4. Combustion air
Tight installation room without adequate combustion air can starve the burner. Look for high-suction events (kitchen exhaust hood running) coinciding with Code 90.
5. Heat exchanger scale
Heavy scale changes burner dynamics enough to push combustion outside the envelope. Descale.
Reset procedure
- Power cycle
- Monitor combustion area for visible issues (yellow flame, soot)
- Escalate to vent inspection if code returns
Bottom line
Code 90 is usually a system-level issue (venting, gas pressure, combustion air) rather than a single failed part. Work through the diagnostic sequence top-down.